It is horribly unkind of the CF SD to try to purloin other people's hard earned days off. She should not even have asked and it was beyond entitled of her to do so.
Loving someone doesn't mean you should let them be unkind and a cheeky fucker to you.
So the CF SD can just do the right and loving thing and treat her stepmother and father with kindnenss and decency and use her own annual leave days on the days when she can't get someone else to look after her own child.
Being a good parent and loving them sometimes means not being a doormat.
You don't have to be a doormat to show you love your adult kid.
And, of course, even if the cheeky fucker SD doesn't have days she can use (although she does) that doesn't, actually matter.
What you don't do, ever, is try to filch other people's very hard earned annual leave days because your kid needs a babysitter. CF SD gets plenty of support already.
What you do instead is figure out a way for your child to be looked after without harassing your stepmother or father and trying to help yourself to their hard earned holiday time.
It's beyond entitled to try to blackmail people into giving up their own annual leave to look after YOUR CHILD.
It's not difficult. It's not complicated. Of course OP must not do this.